Process, voltage, temperature independent switched delay compensation scheme

ABSTRACT

A delay compensation circuit for a delay locked loop which includes a main delay line having a fine delay line comprising fine delay elements and a coarse delay line comprising coarse delay elements, the main delay line being controlled by a controller, the delay compensation circuit comprising: an adjustable fine delay for modeling a coarse delay element, a counter for controlling the adjustable fine delay to a value which is substantially the same as that of a coarse delay element, a circuit for applying a representation of the system clock to the delay compensation circuit, and a circuit for applying the fine delay count from the counter to the controller for adjusting the fine delay line of the main delay line to a value which is substantially the same as that of a coarse delay element of the main delay line.

This application is a continuation application of U.S. Ser. No.09/106,755 filed Jun. 30, 1998.

FIELD OF THE INVENTION

This invention relates to circuits in which fine delay elements areswapped with a coarse delay element during tracking, such as in a delaylocked loop (DLL), and more particularly to a circuit and method forcompensating for tracking differences between switchable coarse and finedelay elements.

BACKGROUND TO THE INVENTION

A digital delay locked loop is generally formed of a phase detectorwhich detects the phase difference between a system clock and a feedbackclock, and causes adjustment of a time delay circuit in the loop whichcauses the DLL output clock to be adjusted to lock with the systemclock. The time delay is generally formed of an adjustable delay line.

Since the delay line is typically adjusted in steps, the finest delayresolution depends on the delay line step increments. In order to holdthe locked condition, the delay line is continuously increased anddecreased in step increments around a lock point, which results ininherent tracking jitter. In order to decrease the jitter, the delayline has been formed of plural coarse delay elements (CDE), forming acoarse delay line, in series with plural fine delay elements (FDE).After power-up of the circuit, the coarse delay line is adjusted, andonce a lock point has almost been determined, the fine delay line isadjusted, which narrows the window around the lock point, to about 25picoseconds, which represents the nominal amount of jitter in a typicalapplication.

One fine delay line element (FDE) consists of 32 steps×25 picosecondsresulting in a time delay of about 0.8 ns, which approximately equalsthe time delay of a single stage of the coarse delay line. Once thedelay locked loop has stabilized to the lock point, the delay line willautomatically compensate for variations in delay caused by changingtemperature and voltage conditions, by varying the fine delay line.

The fine delay line is reset to the halfway point after which it beginstracking.

In case of major drift, adjustments in the fine delay line will overrunits end. In that case, another coarse delay element is switched inseries or an existing coarse delay element is switched out of the coarsedelay line, and at the same time the fine delay line is adjusted tocompensate for the coarse delay increase or decrease to provide the sametotal delay as before. However, now the fine delay line can be usedagain to compensate changes without immediate danger of overrun.

U.S. Pat. No. 5,544,203 invented by Casasanta et al, and U.S. Pat. No.5,604,775 each discuss adjustment of a delay locked loop delay lineusing coarse and fine adjustment. However, none address the problemsolved by the present invention, as will be elaborated below.

It is assumed in the prior art that interchanging (switching) the finedelay line steps for a coarse delay element provides an equal exchangeof delay. Indeed, any differences between the two appear as jitter ofabout 300 ps on the DLL output clock. This amount of jitter wasconsidered to be tolerable, given the prior art primary application ofsingle data rate synchronous dynamic random access memories (SDRAMS).However, with the advent of tighter access time (T_(ac)) specificationsfor double data rate (DDR) SDRAMs which are synchronized to the risingand falling edges of a system clock rather than only to the rising edge,even an amount of jitter of 200 ps-300 ps is becoming intolerable,considering the numerous sources adding to this jitter apart from theDDL, including input clock to data skew, clock duty cycle variations,inaccuracies in the actual input and output buffer delays with respectto its design model, etc.

DLL jitter itself consists of factors such as inherent tracking jitterand supply noise and substrate noise induced jitter. The inherenttracking jitter is caused by the up and down adjustments to the finedelay line while the DLL is in the locked condition, and as describedabove, is a variation equivalent to the delay achieved through a singlestep in the delay line. The jitter caused by switching between thecoarse and fine delay elements caused by the mismatch between theelements is referred to as a switching jitter. This mismatch is highlydependent on the manufacturing process, and thus is hard to predict inthe design stage. As the operating frequencies continue to increase, theswitching jitter can undesirably reduce the data eye significantly. Inaddition, since this switching occurs only infrequently, it isinherently hard to detect during testing and can cause apparentlyrandomly dropped bits when the part is in use in the field.

Analog techniques can be used to achieve a wide range of fine resolutiontracking for various applications. In particular DLLs based on phasemixers have been shown to achieve high fine resolution tracking rangethrough quadrature mixing. However, most analog based DLL designs employsome form of charge pumps for voltage controlled delay lines and as suchthey suffer from a limited resolution of the delay steps since thecontrolling element affects an entire delay line. In addition such DLLsoften require a large acquisition time due to loop bandwidths beinglimited to a small fraction of the clock frequency to ensure stabilityof the loop. This effect also causes a poor jitter performance in analogDLLs.

Furthermore, analog DLL designs are inherently more susceptible to allsources of noise as their control variables (usually voltage) arereduced to achieve finer resolutions. In particular, SDRAMs provide avery noisy environment for analog blocks in form of supply and substratenoise, which when combined with area restrictions in SDRAMs, sometimespreventing adequate implementation of noise prevention techniquesthrough layout, can result in unreliable DLLs in noisy fieldenvironments.

SUMMARY OF THE INVENTION

The present invention provides a circuit and a method of allowing a DLLto track with fine resolution delay elements, while providing a trackingrange much larger than that provided by the fine delay line without thedanger of producing switching jitter. The present invention accomplishesthis by determining substantially the exact amount of fine delay line tocompensate for a coarse delay element. This determination is done byproviding a model of a coarse delay element and indicating to the finedelay line control the amount of fine delay to switch to substantiallyaccurately substitute for the coarse delay element.

In accordance with an embodiment of the invention, a delay compensationcircuit for a delay locked loop which includes a main delay line havinga fine delay segment and a coarse delay segment, the delay compensationcircuit comprising:

(a) a system clock input receiving a system clock;

(b) a first path coupled to the system clock input, the first pathconsisting of a plurality of controllable fine delay elements;

(c) a second path coupled to the system clock input in parallel with thefine delay elements, the second path including a single coarse delayelement;

(d) a phase detector having a pair of inputs coupled to outputs from thefirst and second path and a pair of outputs; and

(e) a counter having inputs coupled to the outputs from the phasedetector and to the system clock, for providing an output result foradjusting the number of fine delay elements in the first path; and

(f) the counter providing said output result to a fine delay counter inthe main delay line when delays of the first and second paths asubstantially equal.

In accordance with another embodiment, a method of compensating for finedelay jitter in a clock driven main delay locked loop which includescoarse and fine delay elements, comprising: in parallel feeding theclock through a first auxiliary delay path having a number of fine delayelements having an adjustable number of fine delay steps, andsimultaneously feeding the clock through a second auxiliary delay pathhaving a further coarse delay element having the same delay as a coarsedelay element of the main delay line, detecting a phase differencebetween outputs of the first and second auxiliary delay paths andcontrolling a counter thereby, adjusting the number of fine delay stepsof the first auxiliary delay path to accurately compensate delay throughthe second auxiliary delay path, and controlling the fine delay in themain delay locked loop from the counter so as to minimize fine delayjitter in the main delay locked loop.

In accordance with another embodiment of the invention, a method ofcompensating for fine delay jitter in a main circuit when switchingbetween a coarse delay element and plural fine delay elements,comprising: providing a model of a coarse delay element in an auxiliaryfine delay path, and providing a signal to a control circuit in the maincircuit from the auxiliary fine delay path indicating a required amountof fine delay to accurately substitute for the coarse delay element inthe main circuit resulting from the model.

In accordance with another embodiment of the invention, a method ofcompensating for fine delay jitter in a main circuit when switchingbetween a coarse delay element and plural fine delay elements isrequired, comprising: providing a model of a coarse delay elementthrough an auxiliary fine delay line, and providing a count from theauxiliary fine delay line to a control circuit in the main circuitindicating a required fine delay to accurately substitute for the coarsedelay element in the main circuit resulting from the model.

In accordance with another embodiment of the invention, a delay linecompensation circuit for a first delay locked loop which includes a maindelay line having a fine delay controlled by a controller, and a coarsedelay which is switched with the fine delay periodically, the delaycompensation circuit comprising: (a) a second delay locked loop whichincludes an auxiliary fine delay line for modelling a coarse delayelement, (b) a counter for controlling the fine delay of the auxiliarydelay line to a value which is substantially the same as that of thecoarse delay element, (c) a circuit for applying a representation of asystem clock to the auxiliary delay line, and (d) a circuit for applyingthe fine delay count of the counter to the controller for adjustment ofthe fine delay of the main delay line to a delay value which is the sameas that of a coarse delay element of the main delay line.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention will be obtained by aconsideration of the detailed description below, in conjunction with thefollowing drawings, in which:

FIG. 1 is a block diagram of a delay locked loop (DLL) in accordancewith the prior art,

FIG. 2 is a schematic diagram of a fine delay element in accordance withthe prior art,

FIG. 3 is a block diagram of a DLL in accordance with an embodiment ofthe present invention,

FIG. 4 is a block diagram of a delay compensation block used in thediagram of FIG. 3 which contains basic elements used to understand thepresent invention, and

FIG. 5 is a block diagram illustrating the delay compensation block ofFIG. 4 in more detail, in accordance with a preferred embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a DLL in accordance with the prior art. A phasedetector 1 receives a system clock (CLK) signal and a feedback clock(FCLK) signal, compares the phase of the FCLK signal with that of theCLK signal, and issues up and down count control signals to a coarseadjust counter 3, and fine adjust counter 5. The up and down signals arealso monitored by a control logic (CTRL LOGIC) circuit 7, which controlscounters 3 and 5.

The outputs of the counters 3 and 5 are coupled to decoders 9 and 11,which decode the counter outputs and apply control signals respectivelyto coarse delay line 13 and to fine delay line 15. The CLK signal iscoupled to an input of the course delay line, and the output of thecoarse delay line is coupled to the input of the fine delay line.Typically the coarse delay line 13 is formed of RC (resistor-capacitor)delay elements and inverters which are selectively connected in serieswith each other by respective multiplexers. Preferably the fine delayline is formed of a circuit which, as shown in FIG. 2, is comprised of abuffer inverter driver 17 followed by a series resistor 19 followed by agroup of binary weighted capacitors 21 which are selectable by binarylogic signals output from the decoder 11. The decoder 11 can be athermometer decoder, which, to select any of 5 binary weightedcapacitors for example, provides a 5 bit logic signal, one bit percapacitor. Thus for each segment of the typically 32 stages of the finedelay element, the decoder 11 will output a 5 bit logic capacitorselection signal. The fine delay line typically consists of plural finedelay elements.

The output of the fine delay line is coupled via a feedback circuit 23,to an input of the phase detector 1, providing the FCLK signal.

In the manner as described earlier, the CTRL LOGIC 7 controls thecounter 3 and fine delay line counter 5 to insert as many coarse, andthen fine, delay elements to lock the CLK and FCLK signals together asclosely as possible. The CTRL LOGIC 7 senses underflow of the fine delayline counter 5 (i.e. the signal to the decoder 11 being all zeros), andthereupon controls the coarse delay line counter 3 to count one down,and controls the fine delay line counter 5 to provide a count signal tothe decoder 11, which is up 32 steps, which compensates for the decreasein one coarse delay line element. The switching described above is thusperformed.

A more detailed description of the circuit is believed to be redundantfor the reason that it is well understood by persons skilled in the art.

As noted earlier, when the switching occurs, if the fine to coarseelement substitution is not accurate due to the elements not beingexactly the same in time delay, switching jitter occurs which is nottolerable for high speed parts.

In accordance with an embodiment of the present invention as shown inFIG. 3, an auxiliary control for the fine counter 5 which provides delaycompensation 25 is included in the aforenoted circuit. In generaloperation, the delay compensation 25 receives the clock signal CLK, (orpreferably a divided version of CLK to reduce power consumption) andprovides a value to the fine counter 5 which indicates the substantiallyexact value of fine control elements to use to compensate for one coarsedelay line element.

FIG. 4 illustrates the basic elements of an intelligent dynamic delaycompensation circuit which determines the number of fine delay stepsrequired to replace a coarse delay element to be switched out of themain delay line such that the delay mismatch is reduced to less than onefine delay step. This circuit allows the DLL to track the system clockCLK by using high resolution fine delay steps, while covering a rangemuch larger than that provided by the fine delay line, without theinherent danger of switching jitter. It also allows fine delay steps ofhigh resolution to be employed without compromising locking time,integrated circuit area and power.

The clock CLK or a divided version thereof (DIV_CLK) is applied via abuffer 27 to two delay paths in parallel, a reference coarse delay pathwhich contains a coarse delay element 29 to be modelled, and a variablefine delay path which contains a controllable number of fine delayelements 31. The outputs of the two paths are coupled to respectiveinputs of a phase detector 33. Outputs of the phase detector 33 are upand down control signals which result from the phase comparison; thesesignals are applied to a counter 35, which also receives the clocksignal CLK or a divided version of the clock. The counter 35 provides acontrol signal for controlling the number of fine delay elements in thefine delay path of the delay line of FIG. 3, which will be stable whenthe delays of both delay paths are substantially equal. This representsaccurately the number of fine delay steps which should be substitutedfor one coarse delay element. The count of the counter 35 at this pointis output to the counter 5 of the main DLL described with reference toFIG. 3. The circuit of FIG. 4 (or the more detailed preferred circuit ofFIG. 5) is represented by element 25 of FIG. 3.

Since the delay elements in the compensation circuit 25 of FIG. 3 aremanufactured simultaneously with those of the delay lines 13 and 15, andare preferably located on the chip in close proximity, they will exhibitsubstantially the same characteristics.

In a preferred embodiment of the present invention the coarse delayelement 29 is an RC based delay well known in the art driven by aninverter, and has a delay of about 0.8 ns. The fine delay element 31consists of 32 fine steps, each of the form shown in FIG. 2, each stepbeing about 25 ps.

In the main delay line of FIG. 3, preferably four fine delay elementsare used to form a fine delay line 15 of +/−64 fine steps, each elementbeing implemented using binary weighted capacitor based RC delayelements. Without the compensation circuit, during switching, one finedelay element or 32 fine steps are substituted by a coarse delayelement.

The phase detector 33 samples the system CLK or delayed CLK (DIV_CLK) atthe rising edge of CLK or DIV_CLK (the output of the coarse delay path29) and indicates by its output up and down signals whether the DIV_CLKis early or late with respect to REF_CLK. The phase detector can be anyimplementation of a meta-stable hardened rising edge triggered Dflip-flop.

The CNTR 35 is preferably a 6 bit up/down binary counter, which is resetto half of its value, i.e. 32 steps. The counter is clocked from theinput clock DIV_CLK and counts up or down depending on the output of thephase detector.

FIG. 5 illustrates a preferred form of the invention, in which two ofthe aforenoted fine delay elements FDE1 and FDE2 (elements 31A and 31B)are shown serially connected. The elements are selected by means of athermometer decoder 37 driven by the output of counter 35.

The coarse delay path also includes additional fine delay elements FD3and FD4 (elements 39A and 39B) which have intrinsic delays which areequal to the minimum delay of the elements 31A and 31B respectively.These are added because one coarse delay interval of a coarse delayelement 13 could be greater than that of one nominal coarse delayelement, and the additional fine delay elements 39A and 39B add to theinterval of CDE 29 to at least equal to the delay interval which isgreater than one nominal delay element.

In operation, the clock input is driven by DIV_CLK, a divided version ofthe main clock, to reduce power consumption. The DIV_CLK is thenbuffered by buffer 27 and is delayed through the above-described twodelay paths. The fine delay elements 39A and 39B have their controlinputs set to 0, so that the path through them includes only theirintrinsic delay.

The fine delay path is comprised of the two fine delay elements 31A and31B which have their control inputs fed by the counter 35 output. Inthis configuration the DIV_CLK is delayed by tCDE+2×tFDE(0) through thecoarse delay path, where tFDE(0) is the intrinsic delay of a fine delayelement, and tCDE is the time delay of a coarse delay element.

DIV_CLK is delayed by tFDE(I)+2×tFDE(0) through the fine delay path,where I is the counter output and can vary from 0 to 64.

The phase detector samples DIV_CLK at the rising edge of REF_CLK andindicates whether DIV_CLK is late or early with respect to REF_CLK. Thecounter uses the phase detector's output to count up if DIV_CLK is earlyand down if DIV_CLK is late. The output of the counter is used by thefine delay elements FDE1 and FDE2 (31A and 31B) to reduce the error onDIV_CLK. Thus I is adjusted by the above-described feedback mechanism sothat tFDE(I)+2×tFDE(0)=tCDE+2×tFDE(0); or in other words, untiltFDE(I)=tCDE. This ensures that the maximum error on DIV_CLK is no morethan tFDE(I), or approximately 25 ps.

Once the error has been reduced to less than one fine delay elementstep, the counter counts one up and one down to stay within the aboveerror range. In this state the circuit is in lock. Once in the lockedstate, I is continually updated as the temperature and voltageconditions change, thereby providing an accurate count for the finedelay elements that ensures minimum mismatch between the coarse delayand fine delay element delays across process parameters and temperatureand voltage drifts.

The output of the counter 35 can be loaded in parallel into anothercounter which can be implemented within the counter 5 of FIG. 3 and canbe used to control the entire fine delay line 15 in the main delay linewhenever there is an overflow condition and switching between coarse andfine elements is required. A control signal can be used to add I to thefine delay line control output of this other counter by simply loading Iinto the other counter or by subtracting I from the other counter byloading the 1's complement of I into the other counter. This can be usedto account for both overflow and underflow conditions.

For example, if the main delay locked loop is tracking by decreasing thedelay and reaches an underflow (all 0's) condition, it must switch out acoarse delay element and add an equivalent amount of fine delaycontrolled through a count in the other counter. Thus in the all 0 orunderflow condition I from the counter 35 is added to the other counter5 by simply loading the count I into the other counter 5. However,during overflow, the other counter 5 contains all 1's and thus thecounter 35 output I must be subtracted from all 1's to get the desiredoperation. This subtraction from all 1's is simply achieved through theloading of 1's complement of I into the other counter.

While the above-described embodiments are digital whereby counters areused to determine the compensated fine delay to be used duringswitching, such compensation can be achieved using other means. Forexample, a phase accumulator can be used instead of the phase detector33 to compensate the delay by using voltage controlled delay elements.The error voltage achieved through this implementation can then be addedor subtracted from the control voltage used to drive a voltagecontrolled delay line.

In addition, the input drive conditions of the two paths feeding thecoarse and fine paths in the structure of FIG. 5 can be matched, and theoutput loading conditions can be established to accurately model theactual operating conditions of the coarse and fine delay elements of themain DLL.

While the above embodiments have been described using the DLL as thecircuit to which they are applied in order to reduce switching jitter,the concepts can be used in other applications that involve trackingdelays with respect to any reference delay path. For example, theinvention can be used in clock recovery circuits, pin timing tuners usedin integrated circuit testers, etc.

A person understanding this invention may now conceive of alternateembodiments and enhancements using the principles described herein. Allsuch embodiments and enhancements are considered to be within the spiritand scope of this invention as defined in the claims appended hereto.

We claim:
 1. A delay compensation circuit for a delay locked loop whichincludes a main delay line having a fine delay segment and a coarsedelay segment, the delay compensation circuit comprising: (a) a systemclack input receiving system clock; (b) a first path coupled to thesystem clock input, the first path consisting of a plurality ofcontrollable fine delay elements; (c) a second path coupled to thesystem clock input in parallel with the fine delay elements, the secondpath including a single coarse delay element; (d) a phase detectorhaving a pair of inputs coupled to outputs from the first and secondpath and a pair of outputs; and (e) a counter having inputs coupled tothe outputs from the phase detector and to the system clock, forproviding an output result for adjusting the number of fine delayelements in the first path; and (f) the counter providing said outputresult to a fine delay counter of in the main delay line when delays ofthe first and second paths are substantially equal.
 2. The delaycompensation circuit as defined in claim 1, wherein the compensationcircuit and the main delay line are located on a chip in close proximityto each other.
 3. The delay compensation circuit as defined in claim 1,wherein the system clock is a delayed version of the system clock fed tothe delay locked loop.